Closed loop control of continuous seam resistance heated forge welding cylinders

ABSTRACT

A signal emitted from a transducer responsive to the relative power applied during a welding process is enhanced electronically and mathematically to be of use in controlling welder operating parameters and/or in rejecting defective welds. Electronically the signal refined to minimize unnecessary noise in the signal and mathematically the signal is modified and analyzed against a standard. In a high speed welding operation automatic means are necessary to assure weld quality and control welder operation since the rapidity with which the welding takes place is too fast for manual readjustment.

BACKGROUND OF THE INVENTION

This invention relates to an apparatus for measuring the relative powerconsumed during a welding process and, in particular, covers anapparatus to be used as a transducer in connection with a Soudronicwelder adapted to weld the longitudinal side seam of a thin metal canbody. Soudronic welders for this type of application have a secondarytransformer rating of 4 to 8 volts and 5000 amps. The welding is ACresistance type in the frequency range of about 50 to 500 Hz with eachalternating waveform producing a power pulse. A traveling electrodebeing a copper wire is positioned between the surfaces to be welded andthe electrode rolls connected to the output terminals of the secondarywinding of the welding transformer. The copper wire is used between eachof the electrodes and the metal surface to be welded and is movedcontinuously in order to prevent deterioration of the weldingelectrodes.

Can bodies are generally hollow cylindrical constructions which areformed along a longitudinal edge into a closed cylinder leaving bothends open. The meeting edges of the cylinder thus formed from a flatblank of material are overlapped for purposes of welding. The blanks arepreferably fashioned from preprinted (lithography), tinplate or tin freesteel chrome-type such as MRT3. Such material presents ranges from 65 to112 pound plate weight per base box which represents a range of 0.007"to 0.0123" in thickness depending upon the application of the containerto be formed from the tinplate and/or tin free steel chrome-type. Awelded side seam is preferable to other forms of side seams such as asoldered can seam or a glued together joint. More particularly, inaerosol containers which must be capable of withstanding up to 200pounds per square inch of internal pressure, a welded longitudinal sideseam has a great many advantages. Similarly, in containers which are ofa particular configuration which is too large to be drawn (as, forexample, a two-piece container is), a welded side seam gives therequisite strength and simplifies the manufacture of such containers asthey are too long or too large for drawing. In other applications it isimportant to have lithograph information on the exterior surface of thecontainers. Quality lithography cannot be applied at high-speed to apreformed drawn container so a container with a manufactured side seamis required.

Hall effect devices have been used in connection with a number oftransducer applications some of which have been applied to weldingmachines see, for example, U.S. Pat. Nos. Noth 3,240,961; Hill3,194,939; Barnhart et al 3,335,258 and Treppa et al 3,389,239. Each ofthe foregoing is designed to use a Hall device in combination with awelder for purposes of current determination. Similarly, the Hood U.S.Pat. No. 3,365,665 shows a Hall transducer which has been used in asystem for measuring current flow in high voltage conductors e.g. powerlines. Assignee of the present invention has a co-pending application ona Hall effect transducer, U.S. Ser. No. 093,855. These arrangements arenot entirely responsive to the condition of the metal to be welded inthat they primarily sense current flow and do not take into account therelative position of the welding electrodes. In the past weldingmonitors using voltage, current or Hall effect measuring transducershave been used to determine the condition of the power flow duringwelding. These techniques have been deficient in that they measure onlyone parameter which makes up the power available between the weldingelectrodes.

Other techniques that have been used as a means of monitoring weldquality do not possess the desired lack of sensitivity to outsideeffects and in most cases measure a parameter that does not totallycharacterize the quality of a weld. For example, monitoring weldingelectrode voltage only insures that a voltage is present that issufficient to produce the necessary heating if all other factors areconstant such as surface resistance and plate integrity. If either ofthese factors vary there will be no indication of it by monitoring thewelding voltage.

As another example, monitoring welding current will yield informationthat insures that each attempt at weld nugget formation has sufficientcurrent available to produce the required heating. However, should theplate weight vary, for example a 10% increase, there is no indicationthat welding current will change significantly since a 10% increase inthickness will result in an insignificant change in bulk materialresistance. However, a 10% increase in thickness can have a significanteffect on the rate of heat dissipation and the amount of metal whichmust be heated to an acceptable temperature. Without a correspondingchange in welding current for a material thickness increase nodetectable information is available on which to act.

Neither voltage nor current monitoring or the combination of the twowill accurately account for the insidious effects of intermittent.variable and unpredicted shunt resistance paths. These can momentarilyalter the current flowing through the desired weld zone and therebyeffect the weld nugget quality without leaving a measurable trace. As asingle measurable parameter the weld forging roll dynamic motion offersa method of singularly monitoring the effect of any or all weldparameter variations and to provide an indicator value with which toadjust the easily altered welding current parameter. Electrode force orvoltage are other control parameters which could be adjusted. In short,monitoring the weld forging roll dynamics appears to be a goodmeasurement tool for the purpose.

In a high-speed operation such as welding thin metal can bodies atseveral hundred per minute with an alternating current welder, theinfluences of input current and voltage as well as ambient temperaturebecomes significant when one is trying to measure small changes in thewelder operating conditions. It is, therefore, the function of thecircuit herein to completely compensate for the aforesaid conditions byproviding an electrode motion responsive transducer which will be usefulin monitoring the electrode forging action used to weld the side seam ofa thin metal container and same will be set forth in the followingsummary.

SUMMARY OF THE INVENTION

The invention uses a physical measuring device to determine rate ofchange in the velocity of the electrode normal to the direction ofmotion of the shell during welding whereby such a measurement can beused in a system for adjusting the welding power and/or rejectingdefective welds. The concept appreciates that welding is a combinationof heat and forging and same can be monitored by variations in theforging under constant force due to changes in the heat generated duringwelding. That is to say that, in areas of high resistance to the flow ofwelding power the heat generated will be greater thus permitting agreater amount of forging with the same amount of force. Consequently,the force on the welding electrodes, if constant, will vary the positionof the electrodes relative to the weld as a function of the power flow.It becomes possible to measure the weld quality then by application of aposition, velocity, or acceleration transducer mounted to an electrode.

Such a transducer will give a dynamic signal which can be looped or fedback to control any of the parameters which will change the weldingpower input. The overall simplicity of this system is appealing in thatconventional transducers can be easily applied to existing equipment andwill give measurable signals that are usable for monitoring and control.

A preferred arrangement of the present invention includes a single axisaccelerometer to monitor the acceleration characteristic of the weldingroll assembly in a Soudronic seam welding system. The system in itssimplest form has an accelerometer attached to the spring loaded spindlefor a welding roll that provides an output signal proportional to thesecond derivative of the vertical displacement versus time curve of thewelding roll assembly or d² x/dt², where x is an unknown displacementdependent on the weld forging roll spring mass dynamics, materialplasticity characteristics and forging roll speed. Consequently, as thespacing between the rolls varies as a function of the heating andforging process, an electrical signal proportional to acceleration isgenerated.

The advantage of this system for monitoring the weld operation is thatin theory the welding roll dynamics should faithfully represent theresult of applying heating and forging force to form a weld nugget.Stated another way, the follow-up motion of the weld forging rolls whichhave a fixed dynamic spring mass system will accurately and repeatablyindicate whether the combination of welding parameters have achieved asuccessful weld nugget formation.

The accelerometer transducer is electrically isolated with a ceramicstandoff riding upon the end of an outer electrode spring spindle whichapplies the forging force. The accelerometer is a single axis instrumentfor generating vertical acceleration time waveform curves as theelectrode rolls are displaced during welding. Changes in the adjustmentof the heat control for the welder are measurable by changes in thosecurves. To calibrate, the welder is run without current flowing. Thisestablishes background vibration not beneficial to the ideal formationof individual welding nuggets, for example, vibrations caused by thefeed and gauging fingers preceding the electrodes and the lap thicknesstransition between can bodies. The low or no current tests alsoindicated the effect low heat has on the dynamic, vertical motioncharacteristics of the outer electrode.

Recognizable and significant difference between the waveforms foracceptable production welds and incomplete welds are measurable. Reducedheating produced less plastic deformation of the steel joint between theelectrodes, thus changing the slopes and the amplitudes of theacceleration-time waveforms. The repeatability of multiple waveformsmade with the same welding schedule is best toward the middle of thewelded seam of a given can. The transducer is sensitive to a very slightchange in the heat control.

The Soudronics heat control is a precise timing device which regulatesthe portion of a half cycle during which welding current is flowingthrough the welding transformer. One hundred percent heat control meansthat welding current flows for the maximum possible time during eachhalf cycle. Delaying the gate signal which triggers conduction through acontrol SCR would result in a shorter welding current pulse in thetransformer secondary, and less heat in the weld nugget. The Soudronicsheat control varies time t in equation:

Welding heat=current squared, times the resistance, times the time t.

The effects of a given heat control setting varies depending upon thecharacteristics of the switching of the SCR and the frequency of thealternating current flowing in the primary of the welding transformer.For example, a difference between 92% and 93% heat would produce a pulseduration change of the order of a fraction of a millisecond, and theaccelerometer can sense the difference in the motion of the electrode.

The accelerometer is a non-intrusive, non-destructive sensor capable ofproviding real time, dynamic information and an electrical signal whichis a function of the formation of every nugget in the seam. The seismicmass in the accelerometer responds to the forging phase of nuggetformation, and consequently it responds to all parameters affectingheating of the weld nugget.

Considering the millions of seamweld nuggets which have been made, onemust conclude that the average performance of the welder and the processare acceptable and that the vast majority of nuggets are properlyformed. In order to improve process efficiency or welded seam qualitythe challenge is in developing a waveform pattern recognition system anddiscrimination strategy which will ignore the good nuggets and seek outthe bad nuggets, i.e., welds. To correlate waveforms of the typegenerated fast enough or long enough to guarantee all cans produced,requires electronic circuitry which considers peak voltage, slope of thevoltage curve, or the area under the voltage curve. An acceptablecriterion for a two millisecond decision is required.

Damping of the accelerometer or filtering the electrical output willattenuate irrelevant vibration in the welding roll assembly. A singleparameter measurement is used to provide feedback information to anegative feedback servo loop control system. The function of the systemis to compare the measured characteristics of weld forging roll dynamicsto a predetermined set point and continuously adjust a welding variablesuch as current and time (I & t) or heat to maintain the preferreddynamic performance.

Closed loop control in this system is extremely desirable since it willreduce process performance variation due to changing welding parameters,lack of objectivity on the part of operators and their inability toeffectively follow the process because of its high speed nature. ASoudronic W.I.M.A. welding system generates many spot welds per secondand this makes it impossible to exercise judgment as to the quality ofeach of these welds and effectively react to make corrections on aspot-by-spot basis.

In a control system, a command or standard signal can be compared to thefeedback signal from the accelerometer. Any deviation or error betweenthese inputs can be used to implement adjustment of the welding heatcontrol setting and/or forging force. The feedback signal is from theweld forging roll accelerometer and indicates the level of heating, weldjoint condition and/or the level of forging action.

Once the required level of forging associated with a good weld has beenestablished as an input command standard level, the process can becontrolled by a closed loop system which will continuously compare theinput command with the feedback information. The parameters which varyunder normal operation include, plate thickness, tin coating weight,forging force, electrode temperature, temper, surface roughness andsurface resistance. The variations due to abnormal conditions include,joint fit up characteristics (overlap), contamination in the weldingmargin, burrs, abnormal shunt current paths and missing weld currentpulses. Each of these parameters will result in a specific response ofthe forging roll accelerometer. Analysis of each will indicate thatfeedback information from the forging roll will be of the correctpolarity or direction to provide an unambiguous control signal.

OBJECTS OF THE INVENTION

It is, therefore, an object of the present invention to provideequipment which responds to the forging process during welding of thinplate at high speeds.

It is a further object of the present invention to provide a systemwhich is instantaneously responsive to the level of resistance betweenthe electrodes of a welder.

It is still a further object of this invention to provide a monitorwhich is responsive to variations in metal plate thickness.

It is yet another object of this invention to provide a technique whichis simple, reliable, low cost and has the capabilities of detectingsmall differences in the metals to be welded.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged cross-sectional schematic representation of aSoudronic welder equipped with an accelerometer and depicting inexaggerated form a lap weld;

FIG. 2 is a microphotograph magnified 150 times of a satisfactory lapjoint weld showing crystalline grain growth in the middle of the joint;

FIG. 3 is a microphotograph magnified 150 times of a faulty weld showingthe line between the lap with very little or no appreciable crystallinegrain growth thereby evidencing in a cold or no weld situation;

FIG. 4 is a schematic block circuit diagram with wave and pulse diagramsshowing the output of the accelerometer and how it is used to regulatethe welder; and

FIG. 5 is a block diagram of an alternate form of loop feedback system.

DETAILED DESCRIPTION OF THE DRAWINGS

Fusion processes such as electric arc, gas flame, or laser weldingdepend upon the flow of molten metal to achieve the bond and to overcomedeficiencies in joint fitup. Resistance seamwelding of steel is like ablacksmith's forge weld. In that process, the object is to minimizeoverheating the metal until molten and to achieve bonding by the plasticdeformation of red to white hot metal. To achieve this deformation, theblacksmith forged with his hammer. The spring loaded, welding rolls of aseams welder do multiple duty. As electrical and thermal conductors theyperform vital heat control functions. At the same time, or only slightlyout of phase, the electrode rolls produce the plastic deformationrequired for proper forging.

The schematic sketch in FIG. 1 shows the preferred location of anaccelerometer 10 at the end of the spring spindle 11. Transverseconstraints 12 control the action of the spring 13 and keep the springspindle 11 moving vertically. The spring 13 has a force which passesthrough the nip of the electrode rolls 15 and 16 for the outer and innerrolls, respectively. The weld nugget 17 being formed at any giveninstant of the welding is a function of the load from the spring and thewelder power pulse. More particularly, a Soudronic ABM 270 (270 Hzfrequency converter) produces 540 half cycle current pulses or weldnuggets per second.

A vertical force vector free body diagram would consist of the spring 13preload acting downward, the mass of the electrode assembly acceleratedby gravity, and the upward reaction force at the outer surface of thelap joint. The motion of the outer electrode roll 15 and theaccelerometer 10 is analogous to that of a car's hub cap as the wheelrolls down a highway. If the road is smooth and level (i.e., no current,no heat), there is no vertical displacement. When sinusoidal weldingpulses (see FIG. 1) vary the heat developed between the electrode rolls15 and 16, the displacement of the roll is like the car wheel traversinga series of uniformly spaced potholes filled with mud. The softer themud and the deeper the hole, the more violent is the bouncing of thewheel and the larger are the vertical acceleration vectors.

Oscilloscope photographs of acceleration-time curves have been madeunder various welding conditions. Without current flowing, the traceproduced resembled a smooth highway, no potholes. The slight ripples inacceleration curves were due to variations in wire thickness, rollbearing eccentricities, or low level background vibrations. Thedifferences in waveshape attributable to no heat (no current), properheat and insufficient heat are obvious in such traces.

A moving copper wire 18 upper and 19 lower is respectively on theoutside and the inside of the can seam overlap. The wire is then used at18 and 19 as a traveling intermediate electrode. To eliminate problemsof electrode contamination and for economy, both sides of the wire areused in the welding process. This is done by means of returning the wireto be used again but to use the other side thereof. The upper weldelectrode roll 15 applies pressure and the current passes through boththe upper and lower electrode welding rolls 15 and 16 into sections ofwire 18 and 19 which are the electrodes. The overlap seam is thus fusedby pulsed electric resistance welding and one spot weld is made afteranother. These pulses generate the weld nuggets 17. The Soudronic welderuses a round continuous length of copper wire which is flattened byprofiling rolls (not shown).

The power supply for the welder is a motor operating off of standardline current which turns an alternator that generates single phase powerat 380 volts and 270 cycles per second. The alternator power is theinput for the primary windings for a transformer coil. This input is notconstant as the Soudronics' power control circuit (not shown) includesan SCR which turns the power on for a portion of each half cycle andturns the power off when the half cycle crosses zero voltage.Consequently, the input power is on for a percentage of the total cycleand provides 540 pulses per second. Each pulse should generatesufficient heat for a proper weld i.e., weld nugget 17.

Turning now to FIG. 2 which is a microphotograph enlarged 150 timestaken longitudinally along a lap joint, it will be noted that the grainsize through the center of the lap joint is much greater. This evidencesproper welding heat and forging pressure as is necessary to produce acontinuous seamless bond. The grain growth is schematically representedin FIG. 1 as a weld nugget and is a cyrstalline area located primarilyat the joint where a weld current pulse occurs. The pulsing is frequentenough to produce a continuous seamless joint but there are areas ofgreater grain growth which are consequence of the varying heat. Thedepiction in FIG. 1 is exaggerated for purposes of understanding and themicrophotograph shown in FIG. 2, is representative of the actual jointat the lap seam. This microphotograph represents a longitudinal sectiontaken through the center of the lap joint and as such the ripples causedby the weld pulses are not immediately apparent along the outer surfaceof the joint. That is to say that, with the magnification and theprocess used to generate this cross-section, it is difficult to show insuch a small portion of a longitudinal side seam weld the actual rippledsurface as exaggerated and depicted in FIG. 1.

In FIG. 3, an enlarged (150 times) microphotograph of a welded seamwhich is unsatisfactory is shown. The weld in this microphotograph ispoor because insufficient heat was available to join the overlappedmetals. As will be noted, there is a line at the middle of the jointwhich shows the complete failure to bond or weld. There is also anotable lack of crystalline growth in the central portion where thelayers are juxtaposed. This depicts clearly a difference which withoutdestructive analysis cannot be readily determined.

It has been found that the waveform generated by the accelerometer 10 ofFIG. 1 varies depending upon the nature of the weld e.g., that shown inFIG. 2 or FIG. 3. More particularly, as the amount of energy put intothe weld increases the slope of the accelerometer spike, indicative ofvertical acceleration will become more steep. In the event there is toomuch energy applied and/or force at the upper electrode 15, the spikewill become so steep that longitudinal microphotographs of weldsproduced will show almost entirely crystalline structure having largegrain areas and rough outer surfaces in that the overlapped metal isseverly worked and spattered. Conversely, in the situation where thepressure and/or energy is inadequate the slope of the spike will be morehorizontal indicating little or no forging during welding. When checkedby microphotgraphs, the result will be like FIG. 3 with perhaps evengreater spacing between the lapped metal which indicates completefailure to form a seam.

In FIG. 1, the accelerometer 10 is shown schematically. However, in thepreferred embodiment a Model EGC-500DS-50 Miniature Heavy DutyAccelerometer made by Entran Devices, Inc., of New Jersey was used. Thistransducer has a compensated operating range of 80° to 180° F. and islinear to ±, less than 1%. The particular unit used in the preferredembodiment has a range of ±50 units of acceleration relative to themotion of the device with a sensitivity norm of 4 mV per unit ofacceleration and a useful frequency of up to 600 Hz.

The accelerometer was installed atop the spindle 11 inside the spring13. In order to electrically isolate it from the power transmitted tothe outer electrode 15, an insulator 20 was included between the spindle11 and the lever arm 21 which is used to carry the outer electrode roll15. More particularly, electrode roll 15 rotates about a central axis 22which is disposed at one end of arm 21 and the other end of arm 21 anaxis 23 carries it for swinging motion relative to the main support forthe welder. Similarly, inner electrode roll 16 is carried for rotationon its axis 24 on the main support of the welder. The difference betweenelectrode outer roll 15 and the inner electrode roll 16 is that theouter 15 is permitted to swing with arm 21 relative to the welder.

The lap joint 25 in FIG. 1 consists of the outer overlap portion 26 ofthe can body and the inner underlap portion 27 of the can body. Theseare brought together by conventional means (not shown), which rolls theflat precut body blank into an individual can tube arranged to have thedesired amount of overlap and positioned to travel between electroderolls 15 and 16. Two feed fingers 28 (only one is shown) push and squarethe can body with respect to the electrode rolls 15 and 16. Can bodiesmay be fed at 30 to 35 meters per minute which will make a 211×604 canat a rate of 180 per minute. Such a can is made out of 75 pound platebeing the common can maker's designation of pounds of steel per basebox. The latter being a fixed area of 31,360 square inches per side ofplate in a base box. The Soudronic's welder will put out between 20 to25 pulses per inch, i.e., spot welds and the speed of the seam weldingis a function of the weight of the plate from which the body isfashioned. At a given seam welding speed, production rate will increaseas can height decreases. Thinner materials will permit higher seamwelding rates. Between each container there is a space whereby the nextadjacent can is approximately 1 to 2 m.m. from the preceding can. Thecorrect distance between adjacent can bodies must be maintained uniformat all times. If the containers are too closely spaced they will hitresulting in either a bad weld at the end or even welding together.Alternatively, if the containers are too far apart there will be a weldbuildup at the longitudinal leading and trailing ends of the side seam.The distance between cans is adjustable by changing the electrode wirespeed and can be easily determined from the weld wire after it haspassed through the electrode rolls. Starting at the left side of FIG. 4,the accelerometer 10 is shown in block diagram form. Above accelerometer10 is shown the type of trace by a seriatim instantaneous transduceroutput which might appear on an oscilloscope were it to be used asherein described. The output of the accelerometer 10 is connected to acalculator circuit which could be programmed to calculate the area undera portion of the accelerometer trace or to approximate the slope of aportion of the accelerometer trace. Such curves are shown adjacent tothe calculator circuit block. A differentiator or integrator can be usedin the calculator circuit.

It is important to appreciate that the trace taken from theaccelerometer should be representative of an average welding pulse. Forthis purpose circuitry below the accelerometer block and calculatorcircuit block are included. More specifically, a trace of the welderpower waveform is used to determine the portion of each pulse to whichthe calculator circuit reacts. More particularly, the output of theSoudronic's welder power supply alternator should be a pure andrepeatable waveform which is synchronized but phase shifted with respectto the pulse depicted by the accelerometer output trace. The powerwaveform is input to a circuit which isolates the preferred portion ofeach cycle for analysis. That is to say that, the timing of when thecalculator circuit operates is controlled by the isolation circuit suchthat an output of the isolation circuit will operate as an on/offcontrol for the calculator circuit.

To be certain that the accelerometer trace is taken during arepresentative portion of the welding of an individual can body anoptically actuated switch is used to signal when the can body isdisposed under the accelerometer 10. It is preferred that readings betaken at a more central portion of the can body such that transients atthe ends of the container are not included. The optical switch ispositioned to signal a pulse squaring circuit when the timing foraccelerometer readings is proper. The squared pulse thus issuing sets ashift register set-reset circuit which controls a shift registerdesigned to take four readings seriatum. Triggering of the shiftregister is also accomplished by a signal from the isolation circuit.Consequently, the shift register reacts in accordance with signals fromthe isolation circuit and the optical switch whereby four readings aretaken during a prescribed portion of a can cycle.

The four readings are sent from the shift register to four circuits forpulse sampling. The four samples obtained are then sent to a sample andhold circuit for each. The output from the calculator circuit is thuscontrolled and evaluated as data obtained in accordance with sample andhold. The four signals once analyzed are independently sent to a summingamplifier which averages the signals and gives a common overall output.The output is thus available for control of a Soudronic welder by meansof an adjustment as described. The output is also available for readingon a meter, or an oscilloscope or as input for a computer and printer.

The signals from the accelerometer can be used to provide a continuoussignal suitable for use in a closed loop control system. In FIG. 5, ablock diagram is shown for an arrangement which can be used toconstantly monitor the welding process in contradistinction to themonitoring technique already disclosed. Examination of the entire sideseam weld for a container is considered advantageous, but the ability todisregard information from unwanted inputs is difficult to overcome.Inputs such as vibrations from the various machine mechanisms or weldingdiscontinuities because of the gap between containers have been majorstumbling blocks. The circuit disclosed in FIG. 5 recognizes theproblems of such inputs by analysis of the entire waveshape from theaccelerometer output. This technique is different from the previouslydiscussed technique in that more than a single side of a waveform orpolarity of the accelerometer output is used.

Computing techniques such as the use of RMS (Root Mean Square)calculations for analysis of the total accelerometer waveform output aremeaningful. The results of such a calculation are a single polarity DCoutput which with filtering yield a visual presentation of the welderperformance. Similarly, such an output can be used in a closed loopcontrol system to adjust welder operation parameters on a slowlychanging basis. The degree of filtering can be tailored to adjust theresponse time for the control loop or visual display. In addition, afiltered output signal can be used as a tracking reference to set thelimits imposed for individual welding pulse analysis.

The use of RMS conversion of the accelerometer signal appears to meetthree primary requirements considered important to weld quantity. Thatis to say that, the signal necessary for visual monitoring of weldingperformance is available. Moreover, a signal necessary for closed loopcontrol of the process is provided. Finally, the reference valuenecessary for tracking an individual weld pulse and thereby detectingquestionable weld nuggests is obtainable. The circuit design blocked outin FIG. 5 allows for easy compensation of the effects of gravity (forwelder operating in a vertical plane) on the weld monitoring system sothat the final output signal is of the form: ##EQU1## where e in_(s)=ƒd² x/dt²

where

x=displacement of weld forging mechanism

t=time

e in_(g) =ƒ(gravational effects)

The output of the computational conversion can be filtered through asimple integrating function such as: ##EQU2## where t=the time constantof the filter circuit that sets the break point frequency of the lowpass filters. Other types of filtering can be used to enhance specificcharacteristics of the signal.

FIG. 5 shows the accelerometer output above the block for theaccelerometer. The two leads of the accelerometer are connected to adifferential amplifier which amplifies their output and rejects thecommon noise in both leads. The output from the differential amplifieris connected to a device which accounts for the effects of gravity. Thedevice is based on an AC coupled amplifier with unity gain which centersthe signal relative to the horizontal axis of the waveform. Thecorrected signal is then sent to a Root Mean Square converter. Suchdevices are useful for measuring electrical signals derived frommechanical phenomena, such as strain, stress, vibration, shock, bearingnoise and acoustical noise. The electrical signals produced by thesemechanical actions are often noisy, non-sinusoidal and superimposed onDC levels. The requirement for true RMS to provide a constant, value andaccurate measurement is satisfied by the converter. The waveform signalbelow the horizontal axis is shifted above the horizontal axis bysquaring, and the output is changed from a waveform to a DC level orvalue for each welding pulse. Those individual values are a function ofthe shape and amplitude of the particular wave. The output from the RootMean Square converter consists of a series of individual DC values whichare representative of some parameter consistent with each welding pulse.It has been found that this parameter can be used as a measure of thesuccessful or unsuccessful operation of the welder.

For comparison and evaluation purposes, the unfiltered output of the RMSconverter is provided to one side of a comparator differential amplifierand a filtered output is provided to the other input of the comparatordifferential amplifier. The filter is basically a low pass unit which isin the nature of an integrating amplifier. A resistance capacitancecircuit sets the time constant for the integration. The comparator usesthe filtered input as one reference against which the unfiltered inputis analyzed. Should an individual DC level of the unfiltered input besubstantially different from the datum established by the waveform ofthe filtered input a signal is transmitted from the comparator to a canrejection mechanism at the right time which automatically winnows thedefective can from the production stream.

Varying the amount of filter changes the waveform and the reference inFIG. 5 to 1 (time constant) or 2 (time constant) is merely illustrative.Other connections to the output of the Root Mean Square converter leadto a welder control filter where the time constant is greater than thatof the can rejection filter. Such a waveform establishes a slower andsmoother rate of fluctuation as a function of the unfiltered signal.This filter may have twice the time constant of the can rejection filterand as such would provide a signal capable of adjusting welderparameters as mentioned herein frequently enough to keep the welderoperating at peak performance. Likewise, a final filter with a stillgreater time constant called "N" can be used to amend the unfilteredsignal sufficiently so that a visual weld quality display can beprovided which will fluctuate with an appropriate frequency to exhibitthe general trend of operation, thus permitting an operator to overseethe ultimate function of the machine.

Consequently, the process of welding can be monitored, recorded and/orcontrolled by means of a simple device which measures and analyzes theactual process of forging during resistance welding. It is, therefore,desired that the invention in its broadest context include all circuitsand transducer devices which operate to measure and evaluate the forgingaction which occurs during automatic welding. The claims which followare intended to include all such arrangements and approaches which willachieve the concept hereinbefore stated.

What is claimed is:
 1. A circuit for monitoring weld quality in anelectrical resistance heating forge welding process including:(a)welding means for providing welding electrical energy and mechanicalforce to a juncture to be welded; (b) an accelerometer transduceroperatively associated with said welding means and held against at leastone surface of the materials forming said juncture for movementtherewith to provide a signal response in the form of a seriatuminstantaneous outputs related to changes due to said process whichinfluence the various portions of said accelerometer transducer signal;(c) electronic calculating means connected to receive said accelerometertransducer signal response for conditioning same so that saidaccelerometer transducer signal response is enhanced and compared with astandard for issuing an output with respect to the difference from saidstandard, and (d) control means responsive to said compared differenceoutput and connected to operatively regulate performance varyingparameters of said welding means.
 2. The circuit of claim 1 wherein saidtransducer output is a signal which varies with time and said electroniccalculating means is responsive to the configuration of said signalwaveform provided by said transducer.
 3. The circuit of claim 2 whereinsaid control means includes an adjustable filter with a variable timeconstant that can be set to regulate the sensitivity of the controlmeans and a servomechanism adaptable to vary relative power consumedduring welding.
 4. The circuit of claim 3 wherein said control meansincludes a timing device adjusted to periodically connect saidtransducer output to said electronic calculating means for monitoringweld quality only at preset positions of the welding process.
 5. Thecircuit of claim 1 wherein said calculating means includes circuitrywhich receives said seriatum instantaneous outputs of the variousportions of said signal and converts each output to a measurable valueindicative of the nature of the conditions transduced.
 6. The circuit ofclaim 5 wherein said circuitry is a Root Mean Square converter whichprovides an average of said outputs.
 7. The circuit of claim 5 whereinsaid modification is by a differentiator that calculates the rate ofchange of a prescribed portion of said transducer output.
 8. A controlapparatus according to claim 5 wherein said electronic calculating meansis an integrator for determining the area under a portion of saidwaveform output and the standard is a prescribed range of values forsaid area which are indicative of satisfactory welder performance. 9.The circuit of claim 5 wherein said circuitry is provided to calculatethe area under a predetermined portion of the curve defined by thewelder input to thereby integrate said transducer output.
 10. A controlapparatus for a pulsing resistance type welder wherein the longitudinalside seam of a hollow cylindrical open ended thin wall container body iswelded comprising:a pair of juxtaposed roller electrodes for supportinga traveling electrode wire of highly conductive material against alapped joint formed by the longitudinal side seam of a hollow tubularmember, a power supply means for providing high frequency pulses ofelectrical energy across said lapped seam as said hollow tubular body iscarried through the nip of said rolls, an accelerometer transducercarried on the axle of one of said rollers for measuring acceleration ofsaid roller towards and away from said lapped seam and providing awaveform responsive to the relative acceleration between said rollersand said lap seam, an electronic circuit means connected to receive theoutput waveform of said accelerometer and refine same by amplification,noise suppression and gravity subtraction, an electronic calculatingmeans designed to analyze said electronic circuit means output anddevelope a modified signal responsive to the character of said waveformand its relative difference with respect to a standard signal, and acontrol means responsive to said difference between said electroniccalculating means signal and said standard for providing a measure to beused to control the relative energy supplied by said welder.
 11. Acontrol apparatus according to claim 10 wherein said electroniccalculating means is a Root Mean Square converter and said standard isprovided by a low pass filtering of said converter output.